I have received the following reply from Cyberpower support: The model can be operated horizontally, but only on the underside with the cooling fins closed. The other side with the ventilation holes must remain free.

The designation is RBP0051 according to the data sheet. But look what it says on the battery. You can find it here: https://www.battery-direct.de/cyberpower-akku-cp1500epfclcd-bd.html Battery for CyberPower PFC SINEWAVE series CP1500EPFCLCD For UPS models: CP1500EPFCLCD Manufacturer: battery-direct© Product group: Replacement battery for CyberPower UPS Product type: UPS battery set (cables and attachments are to be used from the old battery) Battery type: Maintenance-free, sealed brand lead battery with suspended electrolyte (AGM technology). Best high current capability for UPS systems. Directive: APC UPS 2002/95/EC, RoHS compliant Warranty: 2 years (optionally 3 years) Lifetime: Up to 6 years at 25°C Length: 151 mm Width: 65 mm Height: 94 mm Weight: 4.70 kg Search for a compatible battery at CH-shops, e.g. Conrad or distrelec.ch.